1. Field of the Invention
This invention relates generally to coordinate measuring machines, and more particularly to the active error compensation of such machines.
A coordinate measuring machine is a device that can accurately perform three dimensional measurements of very complex parts. The machine basically probes a test part and determines its spatial coordinates. Among the many uses of the machine, the measurements which it takes will allow the manufacture of a duplicate part.
Several types of coordinate measuring machines exist. One type is a bridge machine. The bridge machine includes a rigid base and a bridge-like structure that is supported by and travels on the base. The base includes a flat surface which holds the test part. Suspended from the bridge structure is an elongated member called a ram. The ram can move vertically on and horizontally across the width of the bridge structure. At the end of the ram is a probe. The probe senses when contact has been made with the part. The probe is positionable in different attitudes so that it has access to each exposed side of the part.
The machine operates by moving the bridge structure and ram until the probe touches the test part. The probe then signals that contact has been made, and the coordinates of the probe's position at contact are recorded. The ram or bridge is then moved an incremental distance and the procedure is repeated until all exposed surfaces of the test part have been measured. A computer in the machine derives the exact dimensions of the test part from the coordinate data.
Another type of coordinate measuring device is a gantry machine. The gantry machine is used for inspecting parts too large to fit on the supporting base of a bridge machine.
The gantry machine includes a rigid structure that is supported by vertical columns. The test part rests below the rigid structure. A ram is suspended from the structure and positionable in all three coordinate planes. The ram includes a probe which detects when contact is made with the test part.
A third type of coordinate measuring machine is the cantilever-type configuration. In this type of machine, a ram supporting beam is supported at only one of its ends over the work area.
2. Description of the Prior Art
Most coordinate measuring machines introduce some errors into their measurements. These errors are due to a number of contributing factors. One factor is the non-linearity in the movement of the ram or its supporting structure. In the bridge type machine, the ram may continually twist and tilt from a given reference plane as it is moved over the working area of the base.
Another factor stems from the fact that all coordinate measuring machines require a ram to locate the part under inspection, and that the position of the part, once detected, is measured at a point remote from the actual part location.
For example, in the bridge type coordinate measuring machine, the ram is suspended above the part from a horizontal cross-member of the bridge structure. This cross-member includes a scale which is used to measure the position of the part in one of the coordinate planes. Basically the position of the part is measured at the intersection of the ram with the scale on the cross-member. However, it is the ram's free end (where the probe is mounted) which actually detects the location of the part. If the ram is tilted when contact with the part is made, the apparent position of the part, measured remotely from where the part is situated, will not be true. This inaccuracy is commonly referred to in the machine tool art as Abbe error, and may occur whenever a measurement scale is located remotely from an object being measured.
The cantilever-type configured machine is even more susceptible to Abbe errors than the bridge type. Because the ram supporting beam is supported at only one end, different weight probes will cause the beam to droop at varying angles, which may contribute to the measurement errors.
Various strategies have been suggested to reduce Abbe errors and other forms of errors that degrade the accuracy of the coordinate measuring machine.
One method is commonly referred to as pre-calibrated error compensation. This process is where the error introduced by the machine is measured once initially, and the error data is used subsequently to alter actual test measurements.
Pre-calibrating the coordinate measuring machine can reduce errors significantly and is useful in many applications. However, the process has a number of drawbacks.
First, calibration is usually done off-line, that is, when the machine is not being used for part inspection. This is because the process is extremely slow, requiring that much calibration data be compiled. Typically, pre-calibration error data is taken in incremental steps that must encompass the entire (commonly, three dimensional) working volume of the coordinate measuring machine.
Second, the calibration measurements must be stored in some medium for future retrieval, which may require a significant allocation of computer memory. Typically, the calibration data is stored in a three dimensional matrix memory. The same data must be selectively retrieved according to the position of the probe in the working area of the machine, and used to calculate the true three dimensional probe location.
Third, non-repeatable errors will not be compensated for. Such errors include vibration from surrounding machinery, and changes to the machine's structural geometry due to temperature, humidity and other environmental effects.